I don't think the game ever makes absolutely clear whether siding with the Orcs is an action towards the middle Adanos(Xardas) path or Beliar.

Since you get your spells only from shrines you will be fine because whatever mage-specific skill the rebels can teach you you can also learn in the desert.

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I'd just like to interject here and point out that I'm not going to say anything to spoil the mood, Chief. I'll just float here and watch. Don't mind me, just sitting here, floating and watching, that's me.